Many of today's golf balls include at least a core and a cover. The core may be wound, or it may be compression molded form a slug of uncured or lightly cured elastomer composition comprising a high cis content polybutadiene and a metal salt of an .alpha., .beta., ethylenically unsaturated carboxylic acid such as zinc mono or diacrylate or methacrylate. The molded cores may include metal oxides such as zinc oxide to improve the coefficient of restitution or to increase the core weight. Other materials used in the core composition include compatible rubbers or ionomers, and low molecular weight fatty acids such as stearic acid. Free radical initiator catalysts such as peroxides are admixed with the core composition so that on the application of heat and pressure, a complex curing or cross-linking reaction takes place. Wound cores are generally produced by winding a very large elastic thread around a solid or liquid filled balloon center.
Molded golf ball covers generally include in their compositions ionomeric resins such as those sold by E. I. DuPont de Nemours & Company under the trademark "Surlyn.RTM." and more recently, by the Exxon Corporation (see U.S. Pat. No. 4,911,451) under the trademarks "Escor.RTM." or the tradename "Iotek". These resins have become materials of choice for the construction of golf ball covers over the traditional "balata" (trans polyisoprene, natural or synthetic) rubbers.
The golf balls may be finished by applying one or several finishing coats over the cover. For example, there may be a primer coat and a top coat. Either or both of these coats may be pigmented or clear. Several layers of clear or pigmented coatings may be applied. The clear coatings are applied to golf balls to provide a glossy finish to the ball and to protect any trademark or other indicia which may be stamped or painted onto the ball.
The improvement herein resides generally in the formulation of a urethane clear coat for a golf ball. The clear coat may include a small amount of pigment or dye, yet still remain transparent. The clear coat of the present invention is a two part urethane, essentially comprising a polyol and a polyisocyanate. More specifically, however, the improvement resides in the solvent system included in the clear coating composition. The solvent system permits very high solids coating systems (over 45% solids) to be formulated while still maintaining wet-out of the surface of the golf ball during spray application.
Golf ball clear coatings of the prior art comprise low solids (35% by weight). High solids versions of these coatings are difficult or impossible to apply to the golf balls with the same degree of leveling (i.e., flow out) as the lower solids versions.
It has been found that the difficulties involved in applying the prior art coatings can be overcome by incorporating methylamyl ketone (MAK) into the solvent system. While MAK is used as a solvent for high solids coatings in various industrial applications such as those set forth in U.S. Pat. No. 4,597,998, it is unknown for use as a solvent in preparing golf ball coatings.
The present invention is directed to improved golf ball clear coatings having increased solids content (more than 45% solids) and a desirable measure of wet-out over the prior art clear coatings. Levelling or flow out comparable to that of the lower solids version golf ball coating is also achieved. The coatings are preferably two part urethane and incorporate methylamyl ketone into the solvent system.
These and other features of the invention will be apparent from the following description and from the claims.